Adhesive bonds between substrate surfaces (e.g, wood, glass, metals, ceramics or synthetic resins) by means of synthetic resin-containing adhesives, or coatings of substrates with synthetic resins, should be durable in widely varying weather conditions. In the ideal case, the adhesive strength of the substrate-to-synthetic resin bond under all sorts of stress, such as mechanical or thermal deformation, should be stronger in all instances than the internal strength of the synthetic resin (cohesive strength).
When mounting a synthetic resin-covered crown facing on a metallic denture, it is particularly important to durably bond the synthetic resin to the metal without leaving any gaps in-between the resin and the metal. Deficient bonding results in a premature separation of the facing and in the formation of marginal gaps. Deficient bonding also possibly results in the discoloration of the margin as a result of oxidation of the metal skeleton. Moreover, deficient bonding results in a mechanical irritation of tissue along the gap between the facing and the metallic structure.
Inside the mouth a metal/synthetic resin bond or a ceramic synthetic resin bond is subject to special stress. First, these bondings are subject to physico-mechanical stress which occurs during chewing. Second, these bondings are subject to chemical-biological stress under the influence of saliva, food and pharmaceuticals. Moreover, the bond is subject to additional stress caused by temperature variations occurring in the mouth.
Hitherto, the solutions known in the art either produce bonds that are not sufficiently durable, or they require high apparative investment not normally acceptable in dental laboratories. In addition, these methods require the perfecting and mastering of the procedural steps thereof in order to obtain optimum bonds.
Accordingly, German Offenlegungsschrift 32 11 123 discloses a method for applying a crown facing onto a metallic denture, wherein the metallic crown body is roughened by sand blasting, then dipped several times into a silane-containing ultrasonic bath, and thereafter dried. Afterwards, the facing material is operation has several disadvantages. For instance, only silicon-containing non-precious metal alloys can be employed, and the adhesion which is achieved does not withstand long-term exposure to stress in the mouth.
U.S. Pat. No. 4,364,731 describes the preparation of a coupling layer of inorganic oxides (e.g., silica) which are applied onto the metallic surface by a so-called sputtering apparatus. The oxide layer which is obtained is silanized and thereafter the facing material is applied in a manner known per se. The sputtering process has the effect that the metallic surface to be coated is subject to very high temperatures. Moreover, the method can only be realized with enormous apparative investments in dental laboratories.
A further development of the technique described in the above mentioned U.S. patent is proposed in European patent publication 0 151 233. The European publication discloses that the silica-containing coupling layer is produced with a flame-hydrolysis burner. This bonding layer is silanized, and thereafter the synthetic resin facing material is applied in a manner known per se. In the method disclosed in the European publication, the work is also subjected to relatively high temperatures. Good adhesion is only achieved if all of the apparative parameters are strictly observed. However, the desired results can only be realized with a very high apparative investment.
Furthermore, German Offenlegungsschrift 36 42 290 describes a method for improving the adhesion of synthetic materials to metals, wherein a silica layer is applied onto the metallic surface by coating the metallic surface with silica sols or fine dispersions of ultrafine particulate silicic acid. The thus obtained layers are baked at temperatures of 100.degree. to 800.degree. C. However, in this method, the work is also subjected to high temperatures. Moreover, the adhesive strength which is attained is not sufficient for a durable restoration in the mouth when using the precious metals frequently used in dentistry. Conventional sand blasting with silica (E. Combe, "Zahnarztliche Werkstoffe", Carl Hanser Verlag Munchen-Wien, page 299, 1984; German Offenlegungsschrift 35 31 892 and "Metalloberflache", Vol. 37, page 335, 1983), or conventional sand blasting with alumina (Derwent Abstr., 84-228034/37--average particle size of 20 to 60 .mu.m, and U.S. Pat. No. 4,504,228--particle size of about 150 .mu.m), or surface treatment by centrifugal jet machines using steel grit (A.W. Mallory, Industrie-Lackier-Betrieb, page 223, 1985) only results in a cleaning and roughening of the metallic surface. Although particles may occasionally be catapulted onto the surface (K.-A. van Oeteren, "Korrosionsschutz durch Anstrichstoffe", Vol. 1, page 328, 1980), none of these techniques produce a coupling layer.